This invention pertains to radio receiver circuits and, more particularly, to a circuit that determines the phase error between the carrier of an MSK modulated signal and the local oscillator of a quadrature receiver circuit.
Minimum shift keying (MSK) is a frequency/phase modulation technique which is used to transmit digital data, i.e., binary 1's and 0's. A graph of an arbitrary MSK signal is illustrated in FIG. 1-A in which frequency is plotted on the vertical axis and time on the horizontal axis. Referring to this figure, the transmitter's carrier frequency (w.sub.c) is shifted or "deviated" by a specified amount (dw) in one direction to transmit at binary 1, while the carrier frequency is deviated in the opposite direction for a binary 0. For MSK, the deviation (dw) is 1/4.sup.th the bit rate.
The convention used throughout this specification is that a binary 1 corresponds to a positive deviation (+dw) while a binary 0 corresponds to a negative deviation (-dw). It should be understood, however, that this convention is arbitrary and, in all cases, the opposite convention may be used.
To synchronously detect a received MSK signal, it is necessary for the receiver to recover the carrier from the signal. But the carrier is suppressed (the signal frequency is either at w.sub.c +dw or w.sub.c -dw, but never at w.sub.c), so the receiver must somehow recover the carrier and its phase from the signal. Accordingly, the invention described below provides a means for recovering the carrier phase so that conventional synchronous detection techniques can be employed to demodulate the signal.